The Linux minecraft launcher tries to dynamically load some libraries at runtime. On such library is libnsspem.so used for various TLS purposes.  If for some reason libnsspem.so isn't found (like the package isn't installed) the launcher will silently fail and won't report the issue.

The symptom of this is that the launcher thinks it's offline and it can't log in, can't download, and generally doesn't work. Watching the network activity shows that successfully makes network connections but then immediately closes the connection without performing any TLS handshake.  The launcher then reports it's offline or some other generic download error.

Carefully stepping through thousands of lines of strace output revealed the libnsspem.so load attempts and their failure. This sort of error should be bubbled up to the user so they don't have to resort to Wireshark and strace.